‘Cainong No.5’ onion was used as test material，bulbs of onion were planted in November 28th，and the planting density was divided into 4 treatments，which were 3 700(T1)，4 400 (T2)，5 000 (T3)，6 000 (T4) plant per 667 m2，to discuss the effect of late planted onion bulbs planting density on seed growth，yield and quality.The results showed that，at the index of testing in plant nutrition growth，reproduction growth，seed quality，there was no obvious difference in T1，T2 and T3，the differences of every index in T4 was significant with T1，T2 and T3 and the incidence of T4 was high.In seed production，the yield of T3 was higher than that of T4，T2 and T1，increased production by 23.03%，14.41%，50.28% respectively.Therefore，the planting density of late planted bulbs of onion in T3 with 5 000 plant per 667 m2 was appropriate，the planting density larger or smaller could not achieve goal with high yield of seed，good quality of output.